Since the use of simulation is becoming widespread in traffic engineering practice, questions about the accuracy and reliability of its results need to be addressed convincingly. A major criticism related to this issue is proper calibration of the simulation parameters as well as validation, which is often not done or dealt with in an ad hoc fashion. A complete, systematic, and general calibration methodology is presented for obtaining the accuracy needed in high-performance situations. A technique for automating a significant part of the calibration process through an optimization process is also presented. The methodology is general and is implemented on a selected simulator to demonstrate its applicability. The results of the implementation in two freeway sections of reasonable size and complexity, in which detailed data were collected and compared with simulated results, demonstrate the effectiveness of the manual calibration methodology. For instance, through calibration the average volume correlation coefficient on 21 detecting stations improved from 0.78 to 0.96. Comparable results were obtained with the automated calibration procedure with significant time savings and reduced effort.